1. Field of the Invention
The present invention relates to an image reading apparatus utilizing a light beam.
2. Description of the Related Art
Conventionally, in a fabrication process of a semiconductor, when an alignment between an object to be processed, e.g., a semiconductor wafer and a processing apparatus is performed or a sort of the object is checked, an image of a pattern at a predetermined location of the object is picked up, and alignment and checking of a sort are performed on the basis of the image data.
In such an image reading method of, e.g., an apparatus for reading a sort number etched on a semiconductor wafer, a detection mechanism, e.g., a CCD camera or the like is arranged above an etched position of the sort number of the semiconductor wafer, so that a measurement beam is incident on the etched position, and a reflection beam is sensed by the CCD camera. The image data picked up by the CCD camera is compared with prestored image data to check the sort.
In the conventional image reading method by means of dark field measurement, if a diffuse-reflecting thin film, e.g., an Al film is formed on a semiconductor wafer, considerable irregular reflection occurs, resulting in a considerable decrease in contrast of a measured image. Thus, the sort number etched on the semiconductor wafer is difficult to measure.
In order to prevent a decrease in resolution caused by deteriorated contrast due to irregular reflection beam components, an incident angle of a measurement beam to the semiconductor wafer is reduced and a bright field measurement is performed. In the bright field measurement, however, if a portion to be measured has a small three-dimensional pattern or a small step of the pattern, the judgement is disturbed.